COPLAN-35: Keep Mermaid comments clickable

[aushvin23]

Why

Plan authors and reviewers need to open, reply to, and resolve feedback anchored to architecture diagrams. Asynchronous Mermaid rendering currently removes those anchors after initial page load, leaving visible comment counts with no clickable thread in active reviews.

What changes

• Re-highlight comment anchors after the current Mermaid render batch completes.
• Keep direct-linked threads pending until a stable rendered Mermaid label exists.
• Cover cold page loads, theme-triggered re-renders, and direct comment links with system tests.

Why this approach

This reuses the existing anchor matching and popover behavior on Mermaid's rendered HTML labels. A scoped Stimulus event coordinates diagram completion with comment highlighting without introducing a diagram-specific thread model or a second comment UI.

Risk / rollback

Risk is low and scoped to successfully rendered Mermaid content. Invalid diagrams continue to show their source and existing error state. Reverting this commit restores the previous behavior.

Test plan

• rvm use ruby-3.4.7-portable do bundle exec rspec spec/system/comment_ux_spec.rb:180 spec/system/comment_ux_spec.rb:236
• rvm use ruby-3.4.7-portable do bundle exec rspec (1,132 examples, 0 failures)
• node --check engine/app/javascript/controllers/coplan/mermaid_controller.js
• node --check engine/app/javascript/controllers/coplan/text_selection_controller.js

References

• Original Mermaid review warning

Linked ticket

• COPLAN-35

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